The present invention relates to magnetic storage devices and, more particularly, to a method and apparatus for reading a clock track, that has been written onto a magnetic disk surface of a disk drive, using a magneto-optical clock head. Accordingly, the magnetic disk surface can be enclosed in a head disk assembly (HDA) prior to servo writing and the magneto-optical clock head can be used to read the clock track written on the magnetic disk surface, so that servo writing may be performed outside of a clean room environment (provided that a non-contact type actuator positioning apparatus is also used).
A disk drive system is a data storage device. Among other things, a disk drive system includes a plurality of disks which are mounted for rotation about a common axis. Generally, each of the disks has a pair of disk surfaces which are coated with a magnetic material that is capable of changing its magnetic orientation in response to an applied magnetic field. Data is stored digitally within concentric tracks on one or more of the disk surfaces.
Each disk surface has at least one magnetic transducer associated therewith. Generally, each of the magnetic transducers is attached on the end of an actuator arm. All of the actuator arms, and hence the transducers, are ganged together so that they move over their respective disk surfaces in unison. However, only one transducer is capable of reading data from or writing data to a disk surface at any given time.
During operation of a disk drive, each of the disks are rotated about an axis at a substantially constant rate. To read data from or write data to a disk surface, a magnetic transducer is positioned above a desired track of the disk while the disk is spinning.
Writing is performed by delivering a write signal having a variable current to the transducer while the transducer is held close to the desired track. The write signal creates a variable magnetic field at a gap portion of the transducer that induces magnetic polarity transitions into the desired track. The magnetic polarity transitions are representative of the data being stored.
Reading is performed by sensing the magnetic polarity transitions on a track with the transducer. As the disk spins below the transducer, the magnetic polarity transitions on the track present a varying magnetic field to the transducer. The transducer converts the varying magnetic field into an analog read signal that is then delivered to a read channel for appropriate processing. The read channel converts the analog read signal into a properly timed digital signal that can be further processed and then provided to a host computer system.
The transducer can include a single element, such as an inductive read/write element for use in both reading and writing, or it can include separate read and write elements. Transducers that include separate elements for reading and writing are known as xe2x80x9cdual element headsxe2x80x9d and usually include a magneto-resistive (MR) read element for performing the read function.
As is well known in the art, in order to properly position the transducers with respect to their respective disk surfaces, a disk drive system includes a servo system which uses servo information recorded on one or more of the disk surfaces. In general, there are two main types of servo systems. The first type of servo system is known as a dedicated servo system, where a single dedicated disk surface only includes servo information. The dedicated disk surface cooperates with a dedicated servo transducer, which is ganged together with data transducers, to provide positioning information to the data transducers so that the data transducers may be appropriately positioned with respect to their respective disk surfaces. The second type of servo system is known as a sectored servo system and includes sectors of servo information which are interspersed between sectors of data on each of the disk surfaces. As a transducer flies over its respective disk surface, it periodically obtains positioning information from the sectors of servo information recorded on the disk surface so that it can be properly positioned with respect to the surface.
For the two types of servo systems described above, servo information is either written over an entire disk surface (as in the case of a dedicated servo system) or over periodic sections of a disk surface (as in the case of a sectored servo system). In either case, however, it is crucial that the servo information be written accurately.
Servo information is written during the manufacturing process. The process of writing servo information onto one or more of the disk surfaces is known as servo writing or servo track writing. In most conventional systems, an external device known as a servo track writer (STW), which includes its own transducer (STW transducer or clock head), is used to write a servo clock track onto a disk surface upon which servo information is to be written. The transducers of the disk drive system (as opposed to the STW transducer) are used to write servo information onto one or more of the disk surfaces.
More specifically, the transducers of the disk drive system are xe2x80x9cplacedxe2x80x9d and xe2x80x9cheldxe2x80x9d at an appropriate radial distance from the center of the disk using a variety of well-known techniques, such as by using one of a variety of mechanical push-pin systems. As the STW transducer reads timing information from the servo clock track, the transducers of the disk drive system are instructed to write servo information at a specified radial location on their respective disk surfaces (i.e., the position the transducers are being xe2x80x9cheldxe2x80x9d at) based on the timing information read from the clock track. The transducers are then moved to a different radial location and the process is repeated. Accordingly, servo information is placed the disk surfaces at specified radial distances and is based upon the timing information read by the STW transducer from the servo clock track.
Traditionally, servo writing has been performed in clean room environments because STW transducers, which are used to read the clock track, and mechanical push-pin systems, which are used to position the transducers of the disk drive system, (both of which are external to the disk drive system) require the disk surfaces of the disk drive system to be exposed so that they can perform their respective functions. Hence, clean room environments have been used to prevent contaminants from entering the disk drive system, since contaminants can, among other things, negatively affect: (1) the proper writing of servo information onto the disk surfaces by the transducers of the disk drive; and, (2) the proper reading of the servo clock track by the STW transducer.
Because it is generally relatively expensive to maintain a clean room environment, advances have been made in the servo writing process so that it can be performed outside of a clean room environment. For example, optical push-pin systems have been developed, which permit the transducers of a disk drive system to be positioned without being contacted (e.g., via a laser beam and associated components), so that the positioning aspect of servo writing no longer requires the disk surfaces to be exposed. Additionally, prior to the discovery of the present invention, at least one type of magneto-optical clock head has been developed which permits clock track information to be read without requiring the disk surfaces to be exposed. Accordingly, by using one of the many optical push-pin systems and a magneto-optical clock head, the disk surfaces and transducers of a disk drive system can be sealed (in a HDA) prior to servo writing, so that servo writing can be performed outside of a clean room environment.
The aforementioned-type of magneto-optical clock head is described in U.S. Pat. No. 5,339,204 to James et al. entitled xe2x80x9cSystem and Method for Servowriting a Magnetic Disk Drivexe2x80x9d (hereinafter xe2x80x9cthe ""204 Patentxe2x80x9d). In general, the ""204 Patent describes a method and apparatus for servo track writing outside of a clean room environment. According to the ""204 Patent, a clock track is written onto a disk surface inside a sealed head disk assembly (HDA). The clock track is read by a magneto-optical head through a window in the housing of the HDA, which provides timing information to the transducers of the disk drive so that the transducers may write servo information onto their respective disk surfaces based on the timing information read by the magneto-optical clock head.
As explained in the ""204 Patent, the clock track is read by directing a laser beam onto the clock track located on the surface of a disk and measuring the changes in polarization of the beam as it is reflected off of the disk surface. The polarization changes are representative of changes in the direction of magnetization of the clock track. This phenomenon is known as the longitudinal Kerr effect. Further details of the system and method of the ""204 Patent may be obtained by referring to the ""204 Patent directly. The above synopsis is intended to be a general overview, rather than thorough discussion, of the ""204 Patent.
One of the major drawbacks of the system and method described in the ""204 Patent is that the polarization of the beam may be affected by the birefringence of the window through which it enters and exits the disk drive. Therefore, in practical systems, compensation circuitry is required to compensate for such birefringence so that the clock track may be accurately read.
Another drawback of the system and method of the ""204 Patent is that, because the laser beam does not enter the window near Brewster""s angle, energy may be lost due to reflections off of the window. Accordingly, the reflected signal may be more difficult to detect. Hence, the clock track may not be properly read.
A third drawback of the system and method of the ""204 Patent is that a relatively large number of components are required in order for the system to function properly. Specifically, in the embodiments shown in FIGS. 3a and 3b of the ""204 Patent, among other things, at least two beam splitters (310 and 316) and two photo diode detectors (304, 304) are required for each embodiment. The large number of components required for the system and method of the ""204 Patent may increase manufacturing costs and, hence, the cost of disk drives to consumers.
Accordingly, there is a need to develop a method and apparatus for reading a clock track located on a disk surface that is sealed within a HDA assembly by using a magneto-optical clock head that: (1) is not significantly affected by the birefringence effects of the window through which its laser beam enters, (2) does not lose significant amounts of energy due to reflections off of the window through which its laser beam enters, and (3) does not require a relatively large number of components.
The present invention is designed to overcome the aforementioned problems and meet the aforementioned, and other, needs.
It is an object of the present invention to provide a method and apparatus for reading a clock track located on a disk surface sealed within a HDA using a magneto-optical clock head, so that servo writing may be performed outside of a clean room environment. In contrast to the system described in James et al. which relies on being able to detect changes in polarization of a reflected beam, the present invention makes use of the transverse Kerr effect which relates to changes in the intensity of a reflected beam based upon the magnetization to which its corresponding incident beam is subjected to. Thus, instead of having to compensate for changes in polarization due to the birefringence effects of a window through which an incident beam and its corresponding reflected beam pass, such birefringence effects may be ignored. Furthermore, the incident beam can enter the HDA through the window at Brewster""s angle for the material comprising the window which will minimize reflections off of the window. Accordingly, the present invention: (1) is not significantly affected by the birefringence effects of the window through which the incident laser beam enters and the reflected laser beam exits, (2) does not lose significant amounts of energy due to reflections off of the window through which its laser beam enters, and (3) does not require a relatively large number of components.
Other objects, features and advantages of the invention will be apparent from the following specification taken in conjunction with the following drawings.